1. Field of the Invention
This invention relates to a Josephson junction and to a process for the production thereof.
1. Prior Art
In application of superconductors to electronic devices, it is very important to produce Josephson junctions with a large IcRn product. Because high Tc oxide superconductors have layered crystal structures and short coherent lengths, it is very difficult to produce Josephson junctions of a laminate type. Thus, the recent trend in the fabrication of Josephson junctions is toward the formation of grain boundary on a substrate.
Gross, R. et al propose the use of a substrate which is a laminate of two single crystal layers (Phys. Rev. Lett., 64, 228 (1990)). Dary, K. et al propose the use of a substrate having a stepped portion (Appl. Phys. Lett., 58, 543 (1991)). These methods, which utilize the difference in crystal orientations of the superconductors on both sides of the link, are not satisfactory because of the poor reproducibility of the Josephson junction characteristics, a small IcRn product (below 0.3 mV) and difficulty in forming the link at a desired position.
Ghyselen, B. et al propose a YBaCuO/normal metal/YBaCuO junction produced with the use of a focused ion beam micromachining technique in which a 20 KeV Ga focused ion beam with a diameter of 50 nm is used to form a trench in YBaCuO line provided on a SrTiO.sub.3 substrate. The resulting two superconducting electrodes separated from each other by the trench are then connected by deposition of the normal metal bridge (Physica C, 198, 215 (1992)).
A method is also known which comprises the steps of: irradiating a predetermined portion of a surface of a substrate of a MgO single crystal with a Ga focused ion beam to form a steep slope trench in the substrate; depositing YBaCuO superconductor on the surface of the substrate so that a tunnel barrier is formed in the YBaCuO layer; and patterning the YBaCuO superconductor layer to form a wiring pattern such that the tunnel barrier intersects the wiring pattern. In this method, a superconductor/normal metal/superconductor (SNS) structure is considered to be formed because the crystal orientation of the superconductor in the trench differs from that of the superconductor outside of the trench or because a grain boundary is formed on the trench. With this method, however, the reproducibility of the Josephson junction characteristics is not good and the IcRn product is as small as 0.3 mV.